FIG. 1 illustrates a generic fan, wherein a motor M drives fan blades B. The motor is supported by struts S which extend from an external housing H, often called a shroud.
As discussed later in connection with FIG. 6, the struts S often are designed as vanes, to change the path of air flowing through the fan. Such struts are commonly called stator vanes.
In the prior art, one approach to reducing the torsional vibration is to use struts, or stator vanes, of large cross-sectional area, one of which is shown in FIG. 5. These struts can be arranged radially, as in FIG. 6, or tangentially, as in FIG. 7.
However, the large cross-sectional profile area blocks airflow indicated by the arrows A3 in FIG. 5. This blockage causes a pressure loss, which is counter-productive, because a primary purpose of the fan is to provide an increase in pressure, which induces airflow from the high-pressure region to the low-pressure region.
In addition, these large profile struts cause a pressure disturbance that migrates upstream toward the fan blades. If the fan (not shown) is in close upstream proximity to the struts, as each fan blade (not shown) cuts through the pressure disturbance, a pressure pulse is generated. Consequently, the succession of fan blades cutting the disturbances creates a succession of pressure pulses, which is perceived as a siren-type noise. The tangential orientation of FIG. 7 reduces this noise somewhat
A similar comment applies if the fan is downstream of the struts, wherein the fan blades successively cut the wakes of the struts.
Therefore, while struts of large cross-section can reduce torsional vibration, they cause pressure loss and noise.
Curved stator vanes can be used, as indicated by vane V2 in FIG. 8. These have a smaller cross section, which reduces the problem of a large cross section. They also re-direct tangentially flowing air into a more axial direction which improves system pressure rise performance. However, such stator vanes can exhibit a specific type of torsional vibration.
This problem can be corrected, or reduced, by various cross-bracing schemes, as shown in FIGS. 12A-12C. FIG. 13 illustrates additional cross-bracing schemes, wherein non-radial struts are utilized.
However, these cross-bracing schemes suffer some, or all, of the following problems. One problem is that they increase cost and add mass. In some cases, the cost increase is significant, as when the system is molded from plastic resin, because a more complex mold is then required.
Another problem is that the struts increase pressure loss, and the loss is worsened at the points of intersection between two struts.
Yet another problem is that, depending on the arrangement of the struts, they can interfere with the re-direction indicated in FIG. 8. Effective re-direction of flow creates additional pressure rise which often counters the pressure loss associated with the profile and skin friction losses of the member itself. Thus the reduction of effective re-direction represents a further loss in fan system efficiency.